In automotive production, there is a significant demand for lightweight construction using metal sheet components. The use of thinner sheets is essential in achieving weight reduction. However, serious problems can occur during forming of metal sheet components with thinner sheets since the rigidity of the formed part decreases as sheet thickness decreases. Moreover, the formability of the formed part also decreases with decreasing sheet thickness.
Increasing metal sheet component rigidity while reducing weight can be achieved by substituting steel with aluminium, magnesium, or titanium alloys; or by using a three-dimensional (3D) structured metal sheet. The 3D structured sheet enhances the mechanical properties of components primarily by increasing bending stiffness because of increased inertia (the higher the inertia of the component, the stiffer the sheet). Strain hardening, which occurs during the structuring process, also improves the rigidity of the component.
A 3D structured metal sheet is defined as a sheet of metal with a raised relief from the surface of the metal sheet, such as an embossment. Raising the surface into these bosses or protuberances can be achieved, for instance, by application of pressure against a die roller cut. Another form of making a relief pattern into a sheet of metal is by indentation whereby small surface depressions are made by striking or pressing. In the following description, the terms “embossing” and “embossment” are used for both the process and product of indentation and embossment.
3D structured metal sheet can be produced by rolling, between two rollers with at least one roller having a surface in the form of the desired 3D structure, by embossing between two press plates, or by hydro forming. Rolling is a continuous process and pressing can only be run in a semi-continuous process. These processes normally create high strain-hardening.
Heat shields are classically made from metal sheet material, mainly steel, alloys, or aluminium, the material being used for a supporting sheet, a cover sheet and for insulation. However, other materials like glass fibre, felt material, and special plastic and mineral foams can be used against high temperatures and noise, particularly as the insulation material. For instance, U.S. Pat. No. 5,901,428 discloses examples of 3D structured sheets used in a heat shield. The 3D structure is in the form of pyramidal points formed by embossed dimples, and the so formed sheet is used in a stack of structured sheets to form a heat shield barrier with stand-offs to form air pockets. US 2006/0194025 discloses another example of a multi-layer heat shield with complementary contours or dimples formed in adjacent layers. The dimples are formed with a stamping die.
To increase the effectiveness of the heat shield and reduce the space required for the shield, the metal sheet or stack of sheets may be contoured to closely resemble the shape of the outer surface of, for instance, the exhaust manifold. To provide the desired contour in a metal sheet, the resulting outer metal layer of a heat shield typically includes a number of wrinkles. These wrinkles not only reduce the aesthetic appearance, they are also the place where the fatigue of the product can be observed first.
Most of the known 3D structure patterns used today are made of a repeated single form. U.S. Pat. No. 6,966,402 discloses a pattern with a plurality of dimples formed in a geometric shape selected from a group consisting of a spherical shape, a pyramidal shape, a conical shape, and a trapezoidal shape, and where the dimples are distributed in an offset, uniform rows and columns, or in a random pattern. EP 0 439 046 discloses a 3D pattern in the form of a diamond shaped cross-hatching pattern, which allows the sheet to be stretched and compressed as needed. Also the use of wrinkling or dimpling is disclosed, for instance a plurality of creases or ridges such as in the shape of corrugations. U.S. Pat. No. 6,821,607 discloses the use of knobs having a draped or folded type structure which increases the compression resistance for the individual knobs and therefore, increases the bending strength of the entire sheet of material.
In order to form the embossments for each of the above-noted patterns, the material is stretched around a dimple placed as a stand-alone feature in the overall pattern. By this stretching, material is displaced away from the neutral plane of the flat metal sheet adding to the bending rigidity. If the dimples are placed close enough to each other, then the material between the dimples will be partly offset compared to a neutral plane of the flat metal sheet. That is, the flanks of the embossments will start overlapping. However, the peak of the dimple will always form the highest offset point. Due to the common embossing patterns used and the chosen dimple form, the bending rigidity is optimized in one or two directions of the plane at the expense of other directions in the same plane, for instance, by forming unwanted bending lines, e.g., lines where the metal sheet becomes very easy to bend.